This invention relates to anisotropic or so-called Z-axis electroconductive sheet materials (e.g., films) for mechanically joining and electrically connecting electronic circuit components. The invention is more particularly concerned with new methods of producing such materials and with certain materials so produced. The invention is also concerned with electronic assemblies incorporating such materials.
Z-axis adhesive films are well known and have been used commercially in the electronics industry for a number of years. The underlying concept of these films is really quite simple. Basically, a Z-axis film is composed of a sheet-like, dielectric-resin-based adhesive carrier loaded with conductive particles. The particle loading is kept low enough to avoid the formation of continuous electroconductive paths along the adhesive plane (X,Y plane) of the film, whereby the film is rendered conductive, through the particles, only in its thickness direction (Z-direction).
Applications of Z-axis film adhesives are manifold. Typical examples include the bonding of circuits and components such as liquid crystal displays and surface-mount devices. Other exemplary applications include so-called multi-chip modules and chip-on-board systems.
The most common Z-axis adhesive films are randomly conductive by nature, in that the electroconductive particles are randomly distributed throughout the dielectric adhesive carrier. Such randomly conductive films have traditionally been preferred because they are quite easily made. The conductive particles are simply dispersed in a dielectric-resin-adhesive-based solution, and the resulting mixture is then cast to form a film of the desired thickness.
Traditional random Z-axis films typically utilize conductive particles, usually spheres, of non-bonding metals such as silver, nickel, or gold in a thermosetting resin carrier. A common example is silver-filled epoxy film. Recently, more sophisticated non-bonding conductive particles, such as metal-coated non-conductors, are finding increasing use.
When electronic circuit components are joined with such conventional Z-axis films as these above-described, a film of appropriate dimensions is sandwiched between opposed surfaces of the components and the sandwich is subjected to heat and pressure. The dielectric adhesive film is thus caused to conform to the topography of the opposed component surfaces and to bond the surfaces adhesively. Electrical connections are established by pressure contacts of the non-bonding conductive particles and the electrical contact pads on the opposed component surfaces.
Reliability of the electrical connections requires that the conductive particles be maintained under stress. In practice, however, the connections are subject to failure due to such factors as stress relaxation of the dielectric adhesive film and corrosion of metalized surfaces. Also, there is no guarantee that sufficient conductor will be present where needed, because of the random nature of the system. Moreover, conductor will be present between dielectric portions of the opposed component surfaces, leading to undesirable effects including, for example, reduced breakdown voltage, increased dielectric constant, and signal perturbation at high frequencies.
One proposed approach which can avoid the aforementioned problems involves the use of a non-randomly distributed arrangement of conductive regions which also possess adhesive properties. A non-random or ordered distribution is advantageous in that the presence of conductive regions at all locations requiring the formation of an electrical connection can be guaranteed. Conductive regions having adhesive capability are beneficial in that adhesive electrical contacts are formed with the electrical contact surfaces, thereby avoiding the earlier-mentioned disadvantages associated with pressure-contact connections of non-bonding conductive particles.
Notwithstanding their advantages, ordered Z-axis adhesives have been subject to a significant disadvantage in that the methods heretofore proposed for making such materials are relatively complex and inefficient.